BRUSSELS (Kyodo) -- Economic growth in fast-graying Japan has been "surprisingly" strong if measured per working-age adult, despite a much poorer showing in terms of growth per capita, according to a recent study.

Three economists, including Jesus Fernandez-Villaverde from the University of Pennsylvania, analyzed the gross domestic product of advanced economies per working-age adult aged 15 to 64, instead of growth per capita, which is becoming a "misleading indicator for growth theory" due to those economies' aging population.

"Japan appears as a surprisingly robust economy" when demographics are considered, according to the study, also involving Gustavo Ventura of Arizona State University and Yao Wen of Tsinghua University.

From 2008, when the global financial crisis occurred, to 2019, "the country with the highest growth rate of output per working-age adult in this period is Japan" among the Group of Seven major countries plus Spain, at 1.49 percent, the study said.

The growth rate during the period surpassed the corresponding U.S. rate of 1.34 percent, it said.

Through the 1991-2019 period, however, which covers the economic downturn in Japan following a sharp asset price drop, Japan's growth per working-age adult is weaker at 1.39 percent than the U.S. rate of 1.65 percent and Britain's 1.62 percent, according to the study.

While Japan is known to have a high labor force participation by those aged 65 and over, the participation rate stood at around 25 percent in both 1991 and 2019, the study said.

That of the United States rose from around 11 percent to 20 percent during the same period, suggesting that "Japan's relative success cannot be attributed to an increasing employment rate among its older population," it said.

The study also looked into the statistical impact of more people between 65 and 69 continuing working, but "any suitable redefinition of working age gives us roughly the same result," it said.

Fernandez-Villaverde said Japan's labor productivity is "still relatively low," but economic and financial policies have supported the country's economy and helped achieve the high growth per working-age adult.

While Japan needs to remove some of the problems that have kept its productivity low since the 1990s, increasing the fertility rate is the top priority for the country's future growth, he added.